Thin-diameter coaxial cable and method of producing the same

A small-diameter coaxial cable comprising a central conductor, an insulated covering layer arranged on the outer periphery of the central conductor and having air gaps continuous along the longitudinal direction, and an outer conductor layer arranged on the outer periphery of the insulated covering layer;

• said covering layer including an annular portion covering the outer periphery of the central conductor, one or more columnar portions (ribs) extending outward from the annular portion, the outer conductor layer is arranged to be in contact with the outer periphery of the columnar portions, and one or more air gaps continuous along the longitudinal direction are formed on the inner side of the outer conductor layer.

A method of fabricating a small-diameter coaxial cable, comprising:

• a covering die including a central hole for insertion of the central conductor therethrough and a resin discharge portion having a circular annular portion formed on the outer periphery of the central hole and a plurality of radial slits extending radially outward from the outer periphery of the circular annular portion is used in such a manner that the central conductor is inserted through the central hole while at the same time molding by extruding the melted thermoplastic resin, with a draft, from the resin discharge portion thereby to obtain an intermediate molded component including an inner annular portion covering the outer periphery of the central conductor and a plurality of coupling portions extending outward from the inner annular portion and similar in shape to the die,
• after which the intermediate molded component is introduced to the head of a melt extruder, and the outer annular portion is covered by extrusion on the coupling portions by an annular covering die thereby to form the insulated covering layer having the air gaps,
• after which the outer conductor layer and the protective covering layer are sequentially formed on the outer periphery of the insulated covering layer.

A method of fabricating a small-diameter coaxial cable, that comprising:

• a central conductor covered by an extrusion with the thermoplastic resin melted in annular fashion, with a draft, by an annular covering die thereby to obtain an intermediate molded component having an inner annular portion covering the outer periphery of the central conductor,
• after which using a die including a central hole, an annular portion and a resin discharge portion having a plurality of radial holes extending radially from the inner periphery of the annular portion, the intermediate molded component is inserted through the central hole while extruding the melted thermoplastic resin from the resin discharge portion with a draft thereby to form an outer annular portion and a plurality of coupling portions extending to the center, thereby forming the insulated covering layer having the air gaps,
• after which the outer conductor layer and the protective covering layer are sequentially formed and covered on the outer periphery of the insulated covering layer.

A method of fabricating a small-diameter coaxial cable comprising a central conductor, an insulated covering layer arranged on the outer periphery of the central conductor and having air gaps continuous along the longitudinal direction, an outer conductor layer arranged on the outer periphery of the insulated covering layer and a protective covering layer arranged on the outer periphery of the outer conductor layer, comprising:

• using a die having a central hole for inserting the central conductor therethrough and a plurality of T-shaped split holes arranged adjacently to each other on the outer periphery of the central hole, the central conductor is inserted through the central hole while at the same time extruding the melted resin from the central hole and the T-shaped split holes thereby to form the insulated covering layer having the air gaps continuous along the longitudinal direction on the outer periphery of the central conductor,
• after which the outer conductor layer and the protective covering layer are sequentially formed and covered on the outer periphery of the insulated covering layer.

A method of fabricating a small-diameter coaxial cable, comprising a covering die including a central hole for inserting a central conductor therethrough and a resin discharge portion having an annular portion and a plurality of radial slits extending radially outward from the outer periphery of the annular portion is used in such a manner that the central conductor is inserted through the central hole while at the same time molding by extrusion, with a draft, the melted thermoplastic resin from the resin discharge portion thereby to obtain an intermediate molded component (insulated core) similar in shape to the die and having an inner annular portion covering the outer periphery of the central conductor and a plurality of coupling portions extending outward from the inner annular portion, which intermediate molded component is supplied continuously so that an outer conductor layer is formed by covering a hollow compressed stranded wire or winding a metal foil, a laminate film or the like or covering by extending a copper pipe on the outer periphery of the columnar portions, after which an outer covering layer is formed on the outer periphery of the outer conductor layer.